stochastic error扰动项乃是因exclusion of an important explanatory variable （排除了可作為解釋的變數）or due to incorrect specification of the relationships being examined (cf. model risk)。簡而言之就是模型有些不完整或不恰當。扰动项的一阶序列就是看是否扰动项的出現對後續的誤差造成影響。誤差應該最好沒有時序性。